1. Field of the Invention
The present invention relates to a rechargeable lithium battery which includes a negative electrode containing lithium as active material, a positive electrode containing a lithium-manganese complex oxide as active material, and a nonaqueous liquid electrolyte.
2. Description of Related Art
Manganese dioxide, because of its high discharge potential (vs. Li/Li+) and ability to electrochemically store and release lithium, has been studied for use as positive electrode material of a rechargeable lithium battery.
Manganese dioxide tends to undergo destruction of its crystal structure when subjected to repetitive expansion and shrinkage during charge-discharge cycles. In order for the manganese dioxide to be feasible as the positive electrode material of a rechargeable lithium battery, the stability (charge-discharge cycle characteristics) of its crystal structure during charge-discharge cycles must be improved. For example, a lithium-manganese complex oxide composed of manganese dioxide and Li2MnO3 (See, for example, Japanese Patent Laying-Open No. Sho 63-114064) and a lithium-containing manganese dioxide produced by incorporating lithium into crystal lattice of manganese dioxide (See, for example, Japanese Patent Laying-Open No. Hei 1-235158 (one type of lithium-manganese complex oxide)) are reported as being effective to improve such crystal structure stability of manganese dioxide during charge-discharge cycling.
These lithium-manganese complex oxides exhibit good charge-discharge characteristics and have reached a practically acceptable level as the positive electrode material of a rechargeable battery.
Rechargeable lithium batteries using these lithium-manganese complex oxides for their positive electrode materials are often utilized as a memory backup power source for electronic equipments. However, as technology continues to push up performance and reliability levels of those equipments, they come to show problematically insufficient charge-discharge cycle performance characteristics in such application.
The present invention relates to improvement of such a rechargeable lithium battery containing a lithium-manganese complex oxide as its positive electrode active material, and its object is to provide a rechargeable lithium battery which exhibits excellent charge-discharge performance characteristics based on the improved nonaqueous liquid electrolyte.
In order to attain the above-described object, a rechargeable lithium battery in accordance with the present invention includes a negative electrode containing lithium as active material, a positive electrode containing lithium-manganese complex oxide as active material, and a nonaqueous liquid electrolyte containing a solute and a solvent. Characteristically, the nonaqueous liquid electrolyte further contains trialkyl phosphite.
In the present invention, a reaction occurs between trialkyl phosphite and lithium-manganese complex oxide to produce a phosphorous compound film on the lithium-manganese complex oxide. This film serves to prevent the occurrence of a side reaction between the liquid electrolyte and the lithium-manganese complex oxide during charge-discharge cycles, resulting in providing excellent charge-discharge cycle characteristics. If the proportion by volume of trialkyl phosphite to the total volume of the solvent and trialkyl phosphite is below 0.2%, the film formation may become insufficient. On the other hand, if it exceeds 15%, the phosphorous compound film may be thickened excessively to hinder the charge-discharge process. Thus, the amount by volume of trialkyl phosphite is preferably 0.2-15% of the total volume of the solvent and trialkyl phosphite. Particularly good charge-discharge cycle characteristics are obtained when the amount by volume of trialkyl phosphite is kept within the above-specified range.
Particularly good charge-discharge cycle performance characteristics result when the lithium-manganese complex oxide for use in the present invention is a complex oxide of lithium and manganese into which boron or its compound is incorporated in the form of solid solution. This is because the positive electrode composed of such a composition is prevented from undergoing decomposition during charge so that no decomposition product dissolve out into the nonaqueous liquid electrolyte.
For example, Japanese Patent Laying-Open No. Hei 8-279366 (1996) discloses a useful lithium-manganese complex oxide containing boron in the form of solid solution. Specifically, a ratio of the number of boron to manganese atoms (B/Mn) is 0.01-0.20. A mean valence number of manganese is at least 3.80. This complex oxide can be prepared by a method wherein a mixture of a boron, lithium and manganese compound, in a ratio of numbers of atoms (B:Li:Mn) of 0.01-0.20:0.1-2.0:1, is heat treated at a temperature of 150-430xc2x0 C., preferably of 250-430xc2x0 C., more preferably of 300-430xc2x0 C. If the temperature of heat treatment is below 150xc2x0 C., several problems arise including insufficient progress of reaction and insufficient moisture removal from MnO2. On the other hand, if the heat treatment temperature exceeds 430xc2x0 C., decomposition of MnO2 may be caused to occur to reduce a mean valence number of manganese to less than 3.80. As a result, the boron-containing lithium-manganese complex oxide during charge undergoes a change in electronic state to become unstable to result in the increased tendency thereof to decompose and dissolve into the nonaqueous liquid electrolyte. The heat treatment is preferably performed in the air.
Examples of boron compounds include boron oxide (B2O3), boric acid (H3BO3), metaboric acid (HBQ2), lithium metaborate (LiBO2) and lithium tetraborate (Li2B4O7). Examples of lithium compounds include lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium oxide (Li2O) and lithium nitrate (LiNO3). Examples of manganese compounds include manganese dioxide and manganese oxyhydroxide (MnOOH).
Examples of nonaqueous liquid electrolyte solutes found effective to give good charge-discharge cycle performances include lithium trifluoromethane sulfonimide, lithium pentafluoroethane sulfonimide and lithium trifluoromethane sulfonmethide, which will be later illustrated in the Examples.
Examples of nonaqueous liquid electrolyte solvents found effective to provide good charge-discharge cycle performances are mixed solvents containing at least one organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, xcex3-butyrolactone and sulfolane, and also containing at least one organic solvent selected from the group consisting of 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane, tetrahydrofuran, 1,3-dioxolane, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate. The liquid electolytes containing such mixed solvents exhibit high ionic conductivity. When such mixed solvents are used in combination with a negative electrode composed of an alloy of lithium and aluminum, a film having good ionic conductivity is formed on the negative electrode, resulting in the improved charge-discharge cycle performance characteristics.
The negative electrode contains metallic lithium or other active material capable of electrochemical storage and release of lithium. Examples of active materials capable of electrochemical storage and release of lithium include lithium alloys (such as a lithium-manganese alloy, lithium-aluminum-manganese alloy, lithium-lead alloy, lithium-tin alloy and lithium-silicon alloy), carbon materials such as graphite and coke and the like. Particularly when the negative electrode containing a lithium aluminum alloy is used in combination with the liquid electrolyte containing the mixed solvent according to the present invention, a film having good ionic conductivity is formed on the negative electrode, resulting in the improved charge-discharge cycle performance characteristics.
The present rechargeable battery utilizes a lithium-manganese complex oxide for the positive electrode material and trialkyl phosphite as one constituent of the nonaqueous liquid electrolyte. This combination prevents decomposition of the positive electrode material during charge to result in obtaining excellent charge-discharge cycle performance characteristics. Good charge-discharge cycle performance characteristics can be obtained when both of the positive electrode material and nonaqueous liquid electrolyte specified in the present invention are used in combination, but can not be obtained when either of them is used alone.